Hydration during pregnancy: why water alone isn’t enough

From the first few weeks, the body adapts to pregnancy: blood volume changes, the kidneys filter more, and tissues reorganise to make room for the baby, placenta and amniotic fluid. Water is central to almost all these adjustments. Here is a closer look at what sufficient hydration supports:

Increased blood volume

Maternal blood volume increases gradually throughout pregnancy, partly because the kidneys retain more sodium and water, helping to supply the uterus, placenta and therefore the baby with enough blood. This expansion mainly affects the liquid part of the blood (plasma), which increases by around 40 to 50 %, compared with around 20 to 30 % for red blood cell mass[1]. This partly explains the so-called ‘dilutional’ anaemia often seen during pregnancy, which generally reaches its lowest point between the end of the 2nd trimester and the beginning of the 3rd. Reassuringly, it is generally not serious and then stabilises.

Staying well hydrated helps maintain blood flow as blood volume increases, making the work of the heart and kidneys easier when they are under particular strain during pregnancy.

Amniotic fluid

Amniotic fluid surrounds and protects the baby. It cushions them from knocks, allows them to move, helps regulate their temperature and supports lung development, particularly through the swallowing and breathing movements the baby makes in the womb. From around the middle of pregnancy, this fluid is produced mainly by the baby’s urine[2].

Maternal hydration still plays a role: several clinical studies show that drinking water can measurably increase the volume of amniotic fluid, including in women whose fluid volume was already normal[3]. This is a reason not to neglect hydration, particularly in hot weather or if you are vomiting repeatedly. However, it does not replace medical monitoring if there are concerns about the amount of amniotic fluid.

Transport of nutrients

Blood carries vitamins, minerals, trace elements and oxygen to the mother’s cells and, via the placenta, to the baby, without their blood ever mixing directly. It also carries metabolic waste in the opposite direction, to the kidneys for elimination[4]. The kidneys work particularly hard during pregnancy: their filtration rate rises sharply from the first few weeks to cope with this extra work. By reducing the available blood volume, insufficient hydration can make waste elimination more difficult, particularly when fluid losses are high (vomiting, hot weather).

More comfortable digestion

Progesterone levels rise sharply during pregnancy, relaxing the smooth muscles of the digestive tract. This slows bowel transit, giving the colon more time to reabsorb water from stools and making them harder. This can contribute to constipation and bloating. The growing uterus also puts physical pressure on the intestines and rectum, particularly in the last trimester. Drinking regularly, alongside eating a fibre-rich diet, helps offset this water reabsorption and maintain softer stools and more comfortable bowel movements[5].

Temperature regulation

Pregnancy increases the basal metabolic rate: the body produces around 30 to 35 % more heat than when not pregnant[6]. To release this heat, the body greatly increases blood flow to the skin and the activity of the sweat glands, but its ability to dissipate heat remains relatively limited compared with the increase in heat production. This helps explain why pregnant women often tolerate heat less well and are at greater risk of heatstroke, particularly in summer or during physical activity. Sufficient hydration is essential for these mechanisms, especially sweating, to work properly.

On average: between 1.5 and 2.5 litres per day, or around 8 to 12 glasses of water.

This guide refers to drinks (water, herbal teas, broths) and does not include water from food. Fruit, vegetables, soups and yoghurts naturally contain water and also contribute to your overall hydration during the day. You do not need to calculate exactly how much each glass provides. This figure is a guide, not a target to reach at all costs.

Needs vary according to several factors:

•    Your body size and build,

•    Your activity level,

•    The surrounding temperature,

•    Altitude,

•    Nausea or vomiting.

Conversely, drinking much more than your body needs brings no additional benefit. It is better to spread your drinks throughout the day, using thirst and the indicator below as guides, rather than forcing yourself to drink large amounts ‘just because’.

Here is a simple indicator to help you gauge your hydration level:

•    Clear to pale yellow urine: satisfactory hydration

•    Dark urine: it is time to drink more

This indicator is a practical guide, not an exact measure. Urine colour can also be affected by certain supplements, particularly B vitamins, without this indicating a hydration problem.

1st trimester

In very early pregnancy, the rapid rise in the hormone hCG is associated with how often nausea occurs, affecting a large majority of women to varying degrees. When vomiting is frequent, it causes a loss of fluids and electrolytes that increases the risk of dehydration, sometimes before you clearly feel thirsty.

This is also when urination becomes more frequent, partly due to progesterone and the pressure of the growing uterus on the bladder. It is therefore normal to need to urinate often, and it does not mean you should drink less.

Here are a few helpful tips:

•    Take small sips more often, rather than drinking large amounts at once,

•    Choose cool or lightly flavoured water (lemon, mint), which is often easier to tolerate if you feel nauseous,

•    Herbal teas suitable during pregnancy can help you vary what you drink and prevent ‘drink fatigue’,

•    Don’t wait until you feel thirsty, as nausea can mask thirst.

If you are vomiting very frequently, cannot keep fluids down, or have marked signs of dehydration (a very dry mouth, dizziness, very little urine), it is important to speak to a midwife or doctor promptly. These symptoms may indicate hyperemesis gravidarum, which requires specific medical care.

2nd trimester

Blood volume continues to increase over the weeks, as does the kidneys’ filtration rate. They work harder to eliminate waste from both maternal and foetal metabolism. Nausea often lessens during this period, generally making it easier to drink regularly.

During this period, staying well hydrated helps to:

•    Support blood circulation as it works to supply the uterus and placenta;

•    Limit constipation, which is encouraged by slower bowel transit;

•    Maintain your energy as first-trimester fatigue eases.

3rd trimester

Fatigue, heavy legs, a feeling of water retention: this is often when the body needs the most care. By now, the enlarged uterus puts pressure on the veins in the pelvis and on the vena cava, slowing the return of blood from the legs to the heart. Pregnancy hormones, particularly progesterone, also relax the walls of the veins, encouraging fluid to accumulate in the tissues, especially towards the end of the day.

Drinking less does not improve swelling. Quite the opposite: insufficient hydration can make them worse, as well as making urine more concentrated and increasing the risk of a urinary tract infection, which is already more common during pregnancy.

This water retention is primarily due to physical and hormonal factors. Drinking enough will not eliminate it completely, but staying well hydrated, alongside moving around, raising your legs at the end of the day and lying on your left side, is among the common-sense measures generally recommended to limit discomfort[7].

What do electrolytes do?

Electrolytes are minerals that carry an electrical charge when dissolved in the body’s water and contribute to fluid balance, nerve function and muscle function:

•    Sodium, the main mineral in the fluid outside cells, helps retain water around cells and contributes to the transmission of nerve impulses;

•    Potassium, the main mineral inside cells, counterbalances sodium and plays a role in muscle contraction, including contraction of the heart muscle;

•    Magnesium contributes to hundreds of enzyme reactions, including those involved in energy production and muscle relaxation,

•    Calcium plays a role in muscle contraction, blood clotting and nerve signalling, alongside its well-known role in bone health.

Together, these minerals create a gradient that allows water to move into and out of cells, so the body can use it rather than simply pass it through the kidneys.

Why electrolytes can be useful during pregnancy

Pregnancy brings an increase in blood volume, basal metabolic rate and, for many women, sweating. Needs for water and electrolytes rise alongside these changes.

Some situations make it particularly useful to pay attention to this:

•    Increased sweating (heat, physical activity),

•    Vomiting or diarrhoea, which cause a simultaneous loss of water and minerals,

•    Severe fatigue,

•    Muscle cramps, common in late pregnancy,

•    Feeling as though you drink but never feel properly hydrated.

If fluid losses are high (vomiting, diarrhoea, hot weather), moderately increasing electrolyte intake may improve rehydration. Outside these specific situations, water and a balanced diet are generally sufficient.

A moderate intake of electrolytes may contribute to:

•    Support hydration at a cellular level,

•    Support muscle function, one of the factors discussed in relation to cramping,

•    Support day-to-day energy levels.

Pregnancy cramps have multiple causes, which are still debated. Compression of nerves and blood vessels by the uterus, muscle fatigue and changes in circulation have all been suggested, alongside electrolyte balance. Similarly, a possible link is sometimes raised between dehydration, electrolyte imbalance and Braxton Hicks contractions, the irregular, painless contractions that can occur from the second trimester. The available evidence remains limited and does not establish that taking electrolytes prevents these contractions[8][9]. Above all, it highlights the importance of not neglecting hydration. If you are unsure, or if contractions are regular and close together, discuss this with your midwife or doctor.

Natural sources

For the vast majority of pregnant women, a varied diet is enough to meet electrolyte needs without routinely needing a dedicated drink:

•    Lightly salted broths, also useful when nausea makes solid foods difficult to tolerate;

•    Mineral waters, whose sodium, magnesium or calcium content varies greatly from one water to another (check the label);

•    Potassium-rich fruit and vegetables (banana, avocado, green vegetables);

•    Seeds, nuts and legumes, which are good sources of magnesium.

Drinks with electrolytes

In certain situations (hot weather, regular physical activity, prolonged nausea, marked fatigue), an electrolyte drink can supplement these sources.

Take care when choosing a formula: opt for one without caffeine or excessive sugar. Choose a drink suitable for pregnancy by checking the ingredients list. Ask a healthcare professional if you are unsure. In all cases, these drinks should be used occasionally, alongside water, not as a replacement for it.

Our Hydra Mama stick combines potassium, magnesium, sodium and chloride, with a potassium-to-sodium ratio of 4:1. The formula also includes a touch of coconut water and lemon. Dissolve it in 500 mL to 1 L of water and drink it gradually throughout the day rather than all at once, alongside your usual daily fluid intake.

The sodium content has deliberately been kept moderate. Dietary sodium intake already more than covers the additional needs associated with pregnancy, while potassium and magnesium intakes are often closer to recommended levels without always reaching them. The magnesium in Hydra Mama contributes to a reduction of tiredness and fatigue and to normal energy-yielding metabolism, as well as to the normal functioning of the nervous system and muscles. Potassium contributes to the maintenance of normal blood pressure and normal muscle function.

As with any supplement, it is advisable to seek advice from your midwife or doctor before using it regularly, particularly if you are already receiving treatment or following a diet low in sodium or potassium.